The present invention relates to a technique for mixing and stirring a fluid in a channel by radiation of ultrasound.
The technique for mixing a fluid (in which particles may be incorporated) in a microdevice for microfabrication is essential for achieving chemical microanalysis such as micro TAS. However, in a channel for microfabrication whose cross sectional area is extremely smaller than its length, wherein a solution flows at high speed, a laminar flow easily occurs in the channel. Thus, in order to stir and mix different solutions effectively in the channel, it is necessary to build a special structure in the channel. For example, techniques have been proposed such as bending the channel into a dog-legged shape repeatedly whereby, the direction of the stream of a solution is constantly changed to prevent occurrence of a laminar flow; or a number of blowing-out openings are made in walls of the channel in which sample solutions flow and a reactant agent is sprayed from these openings and mixed (see P. Gravesen at al. Microfluidics: a review, J. Micromech. Microeng. Vol. 3 (1993) pp. 168-182).
Incidentally, it has been known since the 19th century that ultrasound irradiation makes it possible to trap particles in a fluid without contact or cause a liquid to flow. For example, W. L. Nyborg introduced ultrasonic flow phenomena that ultrasound irradiation causes the liquid itself to flow, in the chapter "Acoustic Streaming" of Physical Acoustics Vol. 2B, Ed. W. P. Mason, Academic Press, 1965.
These phenomena have been considered to be caused by a gradient of ultrasound intensity. In order to obtain a larger driving force, it has been known to increase the change in spatial distribution of ultrasonic energy density or enlarge a decrease in ultrasound in a fluid.